1. Field of the Invention
The present invention relates generally to test apparatus and, more particularly, is directed to a test apparatus for testing a metal disc (i.e., stamper) used to manufacture an optical video disc or for testing tone quality of a molded optical video disc.
2. Description of the Prior Art
In an optical video disc, an analog audio signal is of course recorded together with a video signal. Recently, in order to improve tone quality, there is such an optical video disc in which a digital audio signal according to the same format as that of a so-called compact disc is recorded together with a video signal and an analog audio signal.
In a prior-art optical video disc, a video signal and an audio signal are recorded with a frequency allocation shown in FIG. 1.
More specifically, as shown in FIG. 1, a video signal and two-channel stereo audio signals are FM-converted to FM signals by an FM modulator. That is, the video signal is converted to a video FM carrier signal 30 having a center frequency of 8.5 MHz (frequency deviation is 1.7 MHz) and the stereo two-channel audio signals are converted to an audio FM carrier signal 31 (hereinafter, simply referred to as an analog audio signal) comprising one channel having a center frequency of 2.3 MHz and two channel having a center frequency of 2.8 MHz (frequency deviation is .+-.100 kHz). Further, as earlier noted, the two channel audio signal is pulse code modulated (PCM) by 16 bits and then converted to a digital audio signal 33 in a so-called eight- to-fourteen modulation (EFM) manner. This digital audio signal 33 has the same format as that of the compact disc and has the same content as the analog audio signal and is inserted into the band lower than 2 MHz as shown in the frequency allocation of FIG. 1.
The above-mentioned video FM carrier signal 30 is pulse width modulated (PWM) by the analog signal as a square wave signal. When the recording laser light source is turned ON and OFF in response to the signal waveform to which the digital audio signal is added, a recording laser beam is irradiated on a photoresist coated on the glass master disc so that the photoresist is exposed. Thereafter, when the photoresist is developed, concave and convex patterns corresponding to the signal waveform are formed on the glass master disc by the photoresist, thus the master disc being constructed as the recording master disc. This master disc is nickel-plated by an electroforming-process or electroless plating-process and then the nickel-plated portion is removed from the master disc, thereby forming a stamper. Then, on the basis of the stamper, a disc substrate for a video disc formed of a synthetic resin is copied by a predetermined method such as an injection molding process or the like. A metal thin film is formed on this disc substrate by depositing or sputtering a metal such as aluminum (Al) or the like and then a protecting layer is formed on the metal thin film, thus an optical video disc being formed.
FIG. 2 shows a block diagram of a test apparatus for testing a video signal and an audio signal reproduced from a master disc, a stamper or duplicated disc substrate and optical video disc. In the following description, the master disc, the stamper, the disc substrate and the optical video disc will hereinafter be referred to as a disc for simplicity. FIG. 2 shows an example of the test apparatus which tests tone quality of a stamper 2 provided as the aforenoted disc.
The two-channel analog audio signal 31 and the two-channel digital audio signal 33 recorded on the stamper 2 may be supplied from the same source or different sources. In most cases, the same source is employed to record the same content on the stamper 2 and the video signal is of course recorded on the stamper 2.
The stamper 2 is held on a turntable 1 and, the turntable 1 is rotated by a motor 3. An optical pickup 4 comprises a reproducing laser light source, an objective lens for converging a laser beam emitted from the laser light source on the disc, a detecting device for receiving a reflected-back light from the optical disc through the objective lens and the like, though not shown. The optical pickup 4 is moved along the radial direction of the stamper 2 under the control of a feed control circuit 5. The motor 3 and the feed control circuit 5 are controlled by a playback control circuit 6.
When the disc, inter alia, the stamper 2 is reproduced and tested by this test apparatus, the turntable 1 is rotated in the direction opposite to the normal rotational direction. When the master disc, the duplicated video disc and the like are reproduced and tested, the turntable 1 is rotated in the clockwise direction similarly when the standard optical video disc is reproduced.
The detecting device (not shown) of the optical pickup 4 detects the laser beam irradiated on and reflected back from the stamper 2 to provide an RF signal. The RF signal is supplied to and amplified by a first stage amplifier 7. Then, the video signal thereof is supplied to a bandpass filter (BPF) 8, in which it is filtered out to provide the FM video carrier signal 30. The video signal is demodulated by a video demodulator 9, and the demodulated output is supplied to a 1H delay circuit 10 (H represents one horizontal period). The 1H-delayed video signal and the video signal which is not delayed are switched by a switch device 14 and supplied through a video amplifier 15 to a monitor cathode ray tube (CRT) 16, thereby being displayed. The switch device 14 is changed in position in response to a detected output from a dropout detecting circuit 17 which is supplied with the RF signal from the first stage amplifier 7. If the RF signal contains a dropout component, it is compensated for by the previous value. The video demodulated output is decoded by a 24/40-bit code decoder 11 and then displayed on a display device 12. The 24/40-bit code is a time code.
The analog audio signal 31 and the digital audio signal 33 involved in the RF signal are separated by bandpass filters (BPFs) 19 and 24. More precisely, the band of 2 to 3 MHz of the analog audio signal 31 is filtered-out and the band of lower than 2 MHz of the digital audio signal 33 is filtered-out. The outputs of the bandpass filters 19 and 24 are supplied to an FM demodulator 20 and to an EFM decoder 25, in which they are FM-demodulated and decoded, respectively. The outputs from the FM-demodulator 20 and the EFM decoder 25 are supplied to first and second fixed contacts a and b of a switch device 21. An audio amplifier 22 is connected to a movable contact c of the switch device 21 and, an output of the audio amplifier 22 is supplied to a sound emanating device 23 such as a headphone, speaker and the like. A sub-code decoder 26 is supplied with the sub-code from the EFM decoder 25 and a display device 27 displays the thus decoded sub-code.
In the above-mentioned circuit arrangement of the prior art, in order to test the tone quality of the analog audio signal recorded in the stamper 2, the switch device 21 connects its movable contact c to the first fixed contact a to test the analog audio signal in a real time fashion. In the next step, the switch device 21 connects its movable contact c to the second fixed contact b to test the digital audio signal in a real time fashion. This is because the analog audio signal and the digital audio signal are recorded using different recording principles so that failure occurs in different ways, which requires testing the tone qualities thereof independently. Further, since the disc such as the stamper 2 and the like is tested in a real time fashion, a very long period of time, for example, twice the recording time, is needed in order to test the tone quality.